DYNAMICS PROCESSOR TIPS
ã2002 by Eddie
TIP-1: Judging Bassy
Bass is hard to judge by ear. That’s because room acoustics and monitor
idiosyncrasies can create "local" sonic peaks and dips making it hard to
judge note consistency. Simple Limiters like the Teletronics LA-2A (vacuum
tube), The LA-3A (solid-state) and the LA-4 (IC opamp) all feature a mechanical
VU and Optical Gain Reduction that is perfectly suited for Bass, Guitars
First set the Meter Mode switch to read Input or Output — the VU meter
is great for judging consistency of Bass notes. Then meter Gain Reduction,
adjusting the Threshold to "level" the bumps, use more if there are "dips"
are too deep. A gentle RATIO of 2:1 or 4:1 will do the job unless the Bass
is agressively played, such as the "slap" style. What could be easier?
TIP-2: Delicate Under-Things
Conversely, a mechanical VU meter is slow to respond to transients —
percussion instruments have a Fast Attack — so the recording engineer must
be conservative when setting levels for tambourine, hand claps and snare
especially when the destination is analog tape. For example, VU meter response
to any of these instruments may indicate "–5dB," but the actual level could
easily be 6-dB to 12-dB higher. (A clunky tambourine sound with low frequency
artifacts indicates that clipping has occurred.)
When using a VU meter only to judge Gain Reduction, adjust
Threshold until the meter just begins to move, then back off a hair. Be
sure to listen. For a Compressor-Limiter equipped with LEDs, adjust Attack
and Release to their FASTEST settings. Set Threshold for no more than 6dB
of Gain Reduction, select the most aggressive RATIO ("infinite-to-one"
or 20:1) to achieve the desired result — such as getting more "ambience"
from a drum kit by peak limiting a stereo sub mix. Peak limiting
a Stereo Program mix is a bit more complex, you can start aggressive and
then back of the RATIO and threshold. Again, GR meter movement might
just barely be detectable to get the job done.
TIP-3: Reading Tea Leaves
Tip-1 and Tip-2 are examples of RMS Compression and PEAK Limiting, respectively.
In the case of limiters like the LA-2 and LA-3, the Ratio and Response
times are determined by an Optical Device that has a variable response
- faster with conservative Threshold settings (not more than 6dB), slower
with aggressive amounts of GR (6dB or more). Used aggressively the attack
slows to a medium while the release becomes non linear, initially fast
NOTE: These GR examples are based on actual amounts, NOT as displayed
by the meter.
For Dynamics Processing "Speed" and Depth of Gain Reduction are inter-related.
Peak Limiting requires fast response times assuming conservative GR (6dB
max). Otherwise one of the response times must be slower to avoid
annoying modulation distortion. Typically the Attack is fast (5mS
to 10mS) while the release is much longer 100mS and higher). Try
this with Kick Drum, varying the Attack from Fastest to Slowest settings
to hear what seems like an EQ change. Compression begins with low
RATIOs, Medium-to-Slow Attack and Release times along with a soft knee
can make processing transparent.
IF unsure, always start with the slowest settings and a minimal amount
of meter movement. Then speed up the Attack until it "digs" more
into the track. Some VCA compressors provide a pair of LEDs that
assist the user in setting the Threshold as well as the Release Time so
that the processing activity is centered around the "Knee" whether hard
or soft. When effective but transparent processing is required, extending
the Release time helps considerably.
At the risk of being repetitive, any dynamics processor that relies
on a mechanical VU meter can become even more versatile once taking into
account the meter’s slow dynamic response time. This is especially true
for transient instruments, where an almost undetectable deflection of "one-half
dB" on the VU meter (in Gain Reduction mode) might yield 6dB of Peak Limiting.
This is the starting point when trying to extract more meat and ambience
from drum tracks.
TIP-4: Find the Window
Of the four primary dynamics processing topologies — optical (photo-resistor),
variable-mu (vacuum tube), FET (Field-Effect-Transistor) or VCA (Voltage-Controlled
Amplifier: transistor) — ALL have a "window," the optimum range where the
device performs best. ALL also have a maximum amount of gain reduction
as detailed in Table-1. The temptation is to smash a track within
an inch of its life, but it is better to conservatively process twice —
to and from a recording device, for example — rather than aggressively
TIP-5: Understanding The Black
All of these topologies can be considered a three-terminal black box
—Input, Output and Control. Most of the time audio (AC) is converted to
DC, then "shaped" via Resistor-Capacitor (RC) networks to create a Control
Voltage (CV) that can be dynamically manipulated by Attack and Release
pots (variable resistors). The exception are the classic optical Limiters
— the LA-2a, the LA-3a and to a lesser extent the LA-4. The simple act
of charging and discharging a capacitor is a "Time" issue that has a great
effect on the overall dynamic envelope — the sound of the Compressor /
Limiter processing. The emphasis here is that Input and Output circuitry
of many Vintage Classics (Amplifiers and Transformers, for example) are
equal contributors to the magic associated with these boxes.
The Optical Transmitter in the LA-2 and LA-3 is an Electro-luminescent
panel directly driven by audio — the former from a tube and the latter
from a transformer-coupled transistor-pair, most respectively. In this
case, LIGHT is the Control Signal so it is not necessary to create a CV.
In the LA-4, there is a detector (to convert AC to DC) followed by an op-amp
(to manipulate Ratio and Threshold) feeding a transistor to drive the LED.
While there is a capacitor in this circuit, it has more to do with gain
than to create a charge and discharge characteristic.
The addition of Attack, Release, Ratio, Soft Knee and Hard Knee controls
are great when you know how to use them AND when enough metering is provided
to correlate what is heard to what is seen. While for years the trend has
been to add every possible feature to please every possible user, I think
simplicity is one of vintage gear’s "love-factors." Sure, the VU meter
helps, so does the vast real estate required to accommodate the technology
AND make front panels easier to see.